Block of cardiac ATP-sensitive K(+) channels reduces hydroxyl radicals in the rat myocardium

KATP channels are regulators of programmed cell death and targets for the creation of novel drugs against ischemia/reperfusion cardiac injury

BACKGROUND

The use of percutaneous coronary intervention (PCI) in patients with ST-segment elevation myocardial infarction (STEMI) is associated with a mortality rate of 5%-7%. It is clear that there is an urgent need to develop new drugs that can effectively prevent cardiac reperfusion injury. ATP-sensitive K+ (KATP ) channel openers (KCOs) can be classified as such drugs.

RESULTS

KCOs prevent irreversible ischemia and reperfusion injury of the heart. KATP channel opening promotes inhibition of apoptosis, necroptosis, pyroptosis, and stimulation of autophagy. KCOs prevent the development of cardiac adverse remodeling and improve cardiac contractility in reperfusion. KCOs exhibit antiarrhythmic properties and prevent the appearance of the no-reflow phenomenon in animals with coronary artery occlusion and reperfusion. Diabetes mellitus and a cholesterol-enriched diet abolish the cardioprotective effect of KCOs. Nicorandil, a KCO, attenuates major adverse cardiovascular event and the no-reflow phenomenon, reduces infarct size, and decreases the incidence of ventricular arrhythmias in patients with acute myocardial infarction.

CONCLUSION

The cardioprotective effect of KCOs is mediated by the opening of mitochondrial KATP (mitoKATP ) and sarcolemmal KATP (sarcKATP ) channels, triggered free radicals' production, and kinase activation.

The effect of adaptation to hypoxia on cardiac tolerance to ischemia/reperfusion

The acute myocardial infarction (AMI) and sudden cardiac death (SCD), both associated with acute cardiac ischemia, are one of the leading causes of adult death in economically developed countries. The development of new approaches for the treatment and prevention of AMI and SCD remains the highest priority for medicine. A study on the cardiovascular effects of chronic hypoxia (CH) may contribute to the development of these methods. Chronic hypoxia exerts both positive and adverse effects. The positive effects are the infarct-reducing, vasoprotective, and antiarrhythmic effects, which can lead to the improvement of cardiac contractility in reperfusion. The adverse effects are pulmonary hypertension and right ventricular hypertrophy. This review presents a comprehensive overview of how CH enhances cardiac tolerance to ischemia/reperfusion. It is an in-depth analysis of the published data on the underlying mechanisms, which can lead to future development of the cardioprotective effect of CH. A better understanding of the CH-activated protective signaling pathways may contribute to new therapeutic approaches in an increase of cardiac tolerance to ischemia/reperfusion.

Signaling pathways targeting mitochondrial potassium channels

In this review, we describe key signaling pathways regulating potassium channels present in the inner mitochondrial membrane. The signaling cascades covered here include phosphorylation, redox reactions, modulation by calcium ions and nucleotides. The following types of potassium channels have been identified in the inner mitochondrial membrane of various tissues: ATP-sensitive, Ca²⁺-activated, voltage-gated and two-pore domain potassium channels. The direct roles of these channels involve regulation of mitochondrial respiration, membrane potential and synthesis of reactive oxygen species (ROS). Changes in channel activity lead to diverse pro-life and pro-death responses in different cell types. Hence, characterizing the signaling pathways regulating mitochondrial potassium channels will facilitate understanding the physiological role of these proteins. Additionally, we describe in this paper certain regulatory mechanisms, which are unique to mitochondrial potassium channels.

Cellular Electrophysiology of Iron-Overloaded Cardiomyocytes

Iron, the most abundant transition metal element in the human body, plays an essential role in many physiological processes. However, without a physiologically active excretory pathway, iron is subject to strict homeostatic processes acting upon its absorption, storage, mobilization, and utilization. These intricate controls are perturbed in primary and secondary hemochromatoses, leading to a deposition of excess iron in multiple vital organs including the heart. Iron overload cardiomyopathy is the leading cause of mortality in patients with iron overload conditions. Apart from mechanical deterioration of the siderotic myocardium, arrhythmias reportedly contribute to a substantial portion of cardiac death associated with iron overload. Despite this significant impact, the cellular mechanisms of electrical disturbances in an iron-overloaded heart are still incompletely characterized. This review article focuses on cellular electrophysiological studies that directly investigate the effects of iron overload on the function of cardiac ion channels, including trans-sarcolemmal and sarcoplasmic reticulum Ca²⁺ fluxes, as well as cardiac action potential morphology. Our ultimate aim is to provide a comprehensive summary of the currently available information that will encourage and facilitate further mechanistic elucidation of iron-induced pathoelectrophysiological changes in the heart.

The "Goldilocks Zone" from a redox perspective-Adaptive vs. deleterious responses to oxidative stress in striated muscle

Consequences of oxidative stress may be beneficial or detrimental in physiological systems. An organ system's position on the “hormetic curve” is governed by the source and temporality of reactive oxygen species (ROS) production, proximity of ROS to moieties most susceptible to damage, and the capacity of the endogenous cellular ROS scavenging mechanisms. Most importantly, the resilience of the tissue (the capacity to recover from damage) is a decisive factor, and this is reflected in the disparate response to ROS in cardiac and skeletal muscle. In myocytes, a high oxidative capacity invariably results in a significant ROS burden which in homeostasis, is rapidly neutralized by the robust antioxidant network. The up-regulation of key pathways in the antioxidant network is a central component of the hormetic response to ROS. Despite such adaptations, persistent oxidative stress over an extended time-frame (e.g., months to years) inevitably leads to cumulative damages, maladaptation and ultimately the pathogenesis of chronic diseases. Indeed, persistent oxidative stress in heart and skeletal muscle has been repeatedly demonstrated to have causal roles in the etiology of heart disease and insulin resistance, respectively. Deciphering the mechanisms that underlie the divergence between adaptive and maladaptive responses to oxidative stress remains an active area of research for basic scientists and clinicians alike, as this would undoubtedly lead to novel therapeutic approaches. Here, we provide an overview of major types of ROS in striated muscle and the divergent adaptations that occur in response to them. Emphasis is placed on highlighting newly uncovered areas of research on this topic, with particular focus on the mitochondria, and the diverging roles that ROS play in muscle health (e.g., exercise or preconditioning) and disease (e.g., cardiomyopathy, ischemia, metabolic syndrome).

Redox signaling pathways involved in neuronal ischemic preconditioning

There is extensive evidence that the restoration of blood flow following cerebral ischemia contributes greatly to the pathophysiology of ischemia mediated brain injury. The initiating stimulus of reperfusion injury is believed to be the excessive production of reactive oxygen (ROS) and nitrogen (RNS) species by the mitochondria. ROS and RNS generation leads to mitochondrial protein, lipid and DNA oxidation which impedes normal mitochondrial physiology and initiates cellular death pathways. However not all ROS and RNS production is detrimental. It has been demonstrated that low levels of ROS production are protective and may serve as a trigger for activation of ischemic preconditioning. Ischemic preconditioning is a neuroprotective mechanism which is activated upon a brief sublethal ischemic exposure and is sufficient to provide protection against a subsequent lethal ischemic insult. Numerous proteins and signaling pathways have been implicated in the ischemic preconditioning neuroprotective response. In this review we examine the origin and mechanisms of ROS and RNS production following ischemic/reperfusion and the role of free radicals in modulating proteins associated with ischemic preconditioning neuroprotection.

Effect of inhibitors of NADPH oxidase complex and mitochondrial ATP-sensitive potassium channels on generation of dopaminergic neurons from neurospheres of mesencephalic precursors

Reactive oxygen species signaling has been suggested to regulate stem cell development. In the present study, we treated neurospheres of rat mesencephalic precursors with inhibitors of the NADPH oxidase complex and mitochondrial ATP-sensitive potassium (mitoKATP) channel blockers during the proliferation and/or the differentiation periods to study the effects on generation of dopaminergic neurons. Treatment with low doses (100 or 250 μM) of the NADPH inhibitor apocynin during the proliferation period increased the generation of dopaminergic neurons. However, higher doses (1 mM) were necessary during the differentiation period to induce the same effect. Treatment with general (glibenclamide) or mitochondrial (5-hydroxydecanoate) KATP channel blockers during the proliferation and differentiation periods increased the number of dopaminergic neurons. Furthermore, neither increased proliferation rate nor apoptosis had a major role in the observed increase in generation of dopaminergic neurons, which suggests that the redox state is able to regulate differentiation of precursors into dopaminergic neurons.

Nitrite as a mediator of ischemic preconditioning and cytoprotection

Ischemia/reperfusion (IR) injury is a central component in the pathogenesis of several diseases and is a leading cause of morbidity and mortality in the western world. Subcellularly, mitochondrial dysfunction, characterized by depletion of ATP, calcium-induced opening of the mitochondrial permeability transition pore, and exacerbated reactive oxygen species (ROS) formation, plays an integral role in the progression of IR injury. Nitric oxide (NO) and more recently nitrite (NO(2)(-)) are known to modulate mitochondrial function, mediate cytoprotection after IR and have been implicated in the signaling of the highly protective ischemic preconditioning (IPC) program. Here, we review what is known about the role of NO and nitrite in cytoprotection after IR and consider the putative role of nitrite in IPC. Focus is placed on the potential cytoprotective mechanisms involving NO and nitrite-dependent modulation of mitochondrial function.

Reactive oxygen species and mitochondrial adenosine triphosphate-regulated potassium channels mediate helium-induced preconditioning against myocardial infarction in vivo

OBJECTIVES

Helium produces preconditioning by activating prosurvival kinases, but the roles of reactive oxygen species (ROS) or mitochondrial adenosine triphosphate-regulated potassium (K(ATP)) channels in this process are unknown. The authors tested the hypothesis that ROS and mitochondrial K(ATP) channels mediate helium-induced preconditioning in vivo.

DESIGN

A randomized, prospective study.

SETTING

A university research laboratory.

PARTICIPANTS

Male New Zealand white rabbits.

INTERVENTIONS

Rabbits (n = 64) were instrumented for the measurement of systemic hemodynamics and subjected to a 30-minute left anterior descending coronary artery (LAD) occlusion and 3 hours of reperfusion. In separate experimental groups, rabbits (n = 7 or 8 per group) were randomly assigned to receive 0.9% saline (control) or 3 cycles of 70% helium-30% oxygen administered for 5 minutes interspersed with 5 minutes of an air-oxygen mixture before LAD occlusion with or without the ROS scavengers N-acetylcysteine (NAC; 150 mg/kg) or N-2 mercaptoproprionyl glycine (2-MPG; 75 mg/kg), or the mitochondrial K(ATP) antagonist 5-hydroxydecanoate (5-HD; 5 mg/kg). Statistical analysis of data was performed with analysis of variance for repeated measures followed by Bonferroni's modification of a Student t test.

MEASUREMENTS AND MAIN RESULTS

The myocardial infarct size was determined by using triphenyltetrazolium chloride staining and presented as a percentage of the left ventricular area at risk. Helium significantly (p < 0.05) reduced infarct size (23 +/- 4% of the area at risk; mean +/- standard deviation) compared with control (46 +/- 3%). NAC, 2-MPG, and 5-HD did not affect irreversible ischemic injury when administered alone (49 +/- 5%, 45 +/- 6%, and 45 +/- 3%), but these drugs blocked reductions in infarct size produced by helium (45 +/- 4%, 45 +/- 2%, and 44 +/- 3%).

CONCLUSIONS

The results suggest that ROS and mitochondrial K(ATP) channels mediate helium-induced preconditioning in vivo.

Nicorandil improves post-ischemic myocardial dysfunction in association with opening the mitochondrial K(ATP) channels and decreasing hydroxyl radicals in isolated rat hearts

BACKGROUND

Nicorandil has been reported to induce cardioprotection by opening the mitochondrial K(ATP) channels. However, whether nicorandil affects reactive oxygen species is unclear.

METHODS AND RESULTS

The hearts of male Sprague-Dawley rats were excised and perfused on a Langendorff apparatus with Krebs-Henseleit solution with a gas mixture of 95% O(2) and 5% CO(2). 1 mmol/L of nicorandil was given 10 min before ischemia. Left ventricular developed pressure (LVDP, mmHg), +/-dP/dt (mmHg/s) and coronary flow (ml/min) were continuously monitored. All hearts were perfused for a total of 120 min consisting of a 30 min pre-ischemic period, followed by a 30 min global ischemia and 60 min reperfusion with and without 5-hydroxydecanoic acid sodium salt (5-HD), a mitochondrial K(ATP) channel blocker. The concentrations of 2,3-dihydroxybenzoic acid (2,3-DHBA), an indicator of hydroxyl radicals, in the perfusate during reperfusion period were also measured. Nicorandil significantly improved LVDP and +/-dP/dt, and increased coronary flow during reperfusion. Pretreatment with 5-HD abolished the improvement of LVDP and +/-dP/dt, and the increase in coronary flow induced by nicorandil. Nicorandil significantly attenuated the concentrations of 2,3-DHBA during reperfusion, which were restored by 5-HD.

CONCLUSION

Nicorandil is protective against post-ischemic left ventricular dysfunction in association with opening the mitochondrial K(ATP) channels, decreasing hydroxyl radicals and increasing coronary flow in the isolated rat heart.

Blocking cardiac ATP-sensitive K+ channels reduces hydroxyl radicals caused by potassium chloride-induced depolarization in the rat myocardium

The current study examined whether opening of the ATP-sensitive K(+) (K(ATP)) channel can induce hydroxyl free radical (OH) generation, as detected by increases in nonenzymatic formation of 2,3-dihydroxybenzoic acid (DHBA) levels in the rat myocardium. When KCl (4-140mM) was administered to rat myocardium through microdialysis probe, the level of 2,3-DHBA increased gradually in a potassium ion concentration ([K(+)](o))-dependent manner. The [K(+)](o) for half-maximal effect of the level of 2,3-DHBA production (ED(50)) was 67.9microM. The maximum attainable concentration of the level of 2,3-DHBA (E(max)) was 0.171microM. Induction of glibenclamide (10microM) decreased OH formation. The half-maximal inhibitory effect (IC(50)) for glibenclamide against the [K(+)](o) (70mM)-evoked increase in 2,3-DHBA was 9.2microM. 5-Hydroxydecanoate (5-HD, 100microM), another K(ATP) channel antagonist, also decreased [K(+)](o)-induced OH formation. The IC(50) for 5-HD against the [K(+)](o) (70mM)-evoked increase in 2,3-DHBA was 107.2microM. The heart was subjected to myocardial ischemia for 15min by occlusion of left anterior descending coronary artery (LAD). When the heart was reperfused, the normal elevation of 2,3-DHBA in the heart dialysate was not observed in animals pretreated with glibenclamide (10microM) or 5-HD (100microM). These results suggest that opening of cardiac K(ATP) channels by depolarization evokes OH generation.

The direct physiological effects of mitoK(ATP) opening on heart mitochondria

The mitochondrial ATP-sensitive K+ channel (mitoK(ATP)) has been assigned multiple roles in cell physiology and in cardioprotection. Each of these roles must arise from basic consequences of mitoK(ATP) opening that should be observable at the level of the mitochondrion. MitoK(ATP) opening has been proposed to have three direct effects on mitochondrial physiology: an increase in steady-state matrix volume, respiratory stimulation (uncoupling), and matrix alkalinization. Here, we examine the evidence for these hypotheses through experiments on isolated rat heart mitochondria. Using perturbation techniques, we show that matrix volume is the consequence of a steady-state balance between K+ influx, caused either by mitoK(ATP) opening or valinomycin, and K+ efflux caused by the mitochondrial K+/H+ antiporter. We show that increasing K+ influx with valinomycin uncouples respiration like a classical uncoupler with the important difference that uncoupling via K+ cycling soon causes rupture of the outer mitochondrial membrane and release of cytochrome c. By loading the potassium binding fluorescent indicator into the matrix, we show directly that K+ influx is increased by diazoxide and inhibited by ATP and 5-HD. By loading the fluorescent probe BCECF into the matrix, we show directly that increasing K+ influx with either valinomycin or diazoxide causes matrix alkalinization. Finally, by comparing the effects of mitoK(ATP) openers and blockers with those of valinomycin, we show that four independent assays of mitoK(ATP) activity yield quantitatively identical results for mitoK(ATP)-mediated K+ transport. These results provide decisive support for the hypothesis that mitochondria contain an ATP-sensitive K+ channel and establish the physiological consequences of mitoK(ATP) opening for mitochondria.

Effect of nicorandil, a K+ATP -channel opener, on coronary capillary architecture and volume after early myocardial ischemia-reperfusion--a 3-dimensional confocal laser microscopic study--

BACKGROUND

Previous experimental and clinical studies have reported that nicorandil, an adenosine triphosphate (ATP)-sensitive K+ channel opener, exerts a beneficial effect on microvascular function and clinical outcome after myocardial ischemia. The present study assessed whether intravenous administration of nicorandil affects the 3-dimensional (3-D) architectural characteristics of capillaries and their volume after early myocardial reperfusion.

METHODS AND RESULTS

Using the hearts of open-chest anesthetized rats, the left anterior descending artery was occluded for 7 min followed by reperfusion. Nicorandil or saline was infused intravenously during occlusion and reperfusion. The entire coronary microvasculature was filled with contrast medium after the hearts were arrested. Capillaries were observed 3-dimensionally by confocal laser scanning microscopy in both the control area and reperfused area of all samples. The capillary volume fraction was computed from the 3-D images. The reperfused area in both the nicorandil and saline groups showed characteristic architectural changes of the capillaries. Capillary volume fraction in the reperfused area was significantly reduced in saline group, compared with nicorandil group [12.7+/-7.2% vs 18.1 +/-5.3% (p<0.01)].

CONCLUSIONS

Intravenous nicorandil administration has a beneficial effect on capillary damage after reperfusion.

Alterations of the bioenergetics systems of the cell in acute and chronic myocardial ischemia

The aim of the works presented here is to analyze the alterations induced by acute ischemia-reperfusion and chronic ischemia on mitochondrial function, in relation to alterations on heart function. Parameters of mitochondrial function were assessed on skinned fibers coming from isolated perfused rat hearts. The effects of chronic ischemia were studied on a rat model of left descending coronary artery stenosis. Two key events observed after acute ischemia-reperfusion and chronic ischemia are the decrease (or the loss) of the stimulatory effect of creatine and the alteration of outer mitochondrial permeability to cytochrome c and ADP. Taken together, these effects indicate the alteration of the intermembrane space architecture leading to the loss of intracellular adenine nucleotides compartmentation and possibly of functional coupling of mitochondrial creatine kinase and adenine nucleotide translocase. These alterations result in the impairment of intracellular energy transfer (channeling) from mitochondria to ATP-utilizing sites located in the cytosol. This may play a significant role in ischemic injury and alterations in heart function. We show that these effects were prevented by effective cardioprotective strategies like ischemic preconditioning or pharmacological preconditioning by perfusion of mitochondrial ATP-sensitive potassium channel openers. We hypothesize that an open mitochondrial ATP-sensitive potassium channel during ischemia maintains the tight structure of the intermembrane space that is required to preserve the normal low outer membrane permeability to ADP and ATP.

Difference in the cardioprotective mechanisms between ischemic preconditioning and pharmacological preconditioning by diazoxide in rat hearts

BACKGROUND

Recent studies have implicated the opening of mitochondrial K(ATP) (mitoK(ATP)) channels and the production of reactive oxygen species (ROS) in the cardioprotective mechanism of ischemic preconditioning (IPC).

METHODS AND RESULTS

The involvement of mitoK(ATP) channels and ROS in the cardioprotective effects of both IPC and the mitoK(ATP) channel opener diazoxide (DZ) was investigated in ischemic/reperfused rat hearts. The effects of IPC and DZ on myocardial high-energy phosphate concentrations and intracellular pH (pH(i)) were also examined using (31)P nuclear magnetic resonance spectroscopy. Although both the mitoK(ATP) channel inhibitor 5-hydroxydecanoate and the antioxidant N-acetylcysteine abolished the postischemic recovery of contractile function by DZ, neither of them inhibited that by IPC. IPC attenuated the decline in pHi during ischemia, but DZ did not (6.28+/-0.04 in IPC, p<0.05, and 6.02+/-0.05 in DZ vs 6.02 +/-0.06 in control hearts). DZ, but not IPC, reduced the decrease in ATP levels during ischemia (ATP levels at 20-min ischemia: 26.3+/-3.4% of initial value in DZ, p<0.05, and 8.1+/-3.0% in IPC vs 15.1+/-1.3% in control hearts).

CONCLUSIONS

These results suggest that DZ-induced cardioprotection is related to ROS production and reduced ATP degradation during ischemia, whereas attenuated acidification during ischemia is involved in IPC-induced cardioprotection, which is not mediated through mitoK(ATP) channel opening or ROS production.

Reactive oxygen species as mediators of signal transduction in ischemic preconditioning

Ischemic preconditioning (IPC) is a most powerful endogenous mechanism for myocardial protection against ischemia/reperfusion injury. It is now apparent that reactive oxygen species (ROS) generated in the mitochondrial respiratory chain act as a trigger of IPC. ROS mediate signal transduction in the early phase of IPC through the posttranslational modification of redox-sensitive proteins. ROS-mediated activation of Src tyrosine kinases serves a scaffold for interaction of proteins recruited by G protein-coupled receptors and growth factor receptors that is necessary for amplification of cardioprotective signal transduction. Protein kinase C (PKC) plays a central role in this signaling cascade. A crucial target of PKC is the mitochondrial ATP-sensitive potassium channel, which acts as a trigger and a mediator of IPC. Mitogen-activated protein (MAP) kinases (extracellular signal-regulated kinase, p38 MAP kinase, and c-Jun NH(2)-terminal kinase) are thought to exist downstream of the Src-PKC signaling module, although the role of MAP kinases in IPC remains undetermined. The late phase of IPC is mediated by cardioprotective gene expression. This mechanism involves redox-sensitive activation of transcription factors through PKC and tyrosine kinase signal transduction pathways that are in common with the early phase of IPC. The effector proteins then act against myocardial necrosis and stunning presumably through alleviation of oxidative stress and Ca(2+) overload. Elucidation of IPC-mediated complex signaling processes will help in the development of more effective pharmacological approaches for prevention of myocardial ischemia/reperfusion injury.

Early biochemical and histological alterations in rat corticoencephalic cell cultures following metabolic damage and treatment with modulators of mitochondrial ATP-sensitive potassium channels

The present study was aimed at characterizing alterations of the nucleotide content and morphological state of rat corticoencephalic cell cultures subjected to metabolic damage and treatment with modulators of mitochondrial ATP-dependent potassium channels (mitoK(ATP)). In a first series of experiments, in vitro ischemic changes of the contents of purine and pyrimidine nucleoside diphosphates and triphosphates were measured by high performance liquid chromatography (HPLC) and the corresponding histological alterations were determined by celestine blue/acid fuchsin staining. As an ischemic stimulus, incubation with a glucose-free medium saturated with argon was used. Ischemia decreased the levels of adenosine, guanine and uridine triphosphate (ATP, GTP, UTP) and increased the levels of the respective dinucleotides ADP and UDP, whereas the GDP content was not changed. Both 5-hydroxydecanoate (5-HD) and diazoxide failed to alter the contents of nucleoside diphosphates and triphosphates, when applied under normoxic conditions. 5-HD (30 microM) prevented the ischemia-induced changes of nucleotide and nucleoside levels. Diazoxide (300 microM), either alone or in combination with 5-hydroxydecanoate (30 microM) was ineffective. Pyruvate (5 mM) partially reversed the effects of ischemia or ischemia plus 2-deoxyglucose (20mM) in the incubation medium. Diazoxide (300 microM) and 5-HD (30 microM) had no effect in the presence of pyruvate (5mM) and 2-deoxyglucose (20mM). Staining the cells with celestine blue/acid fuchsin in order to classify them as intact, reversibly or profoundly injured, revealed a protective effect of 5-HD. When compared with 5-HD, diazoxide, pyruvate and 2-deoxyglucose had similar but less pronounced effects. In conclusion, these results suggest a protective role of 5-hydroxydecanoate on early corticoencephalic nucleotide and cell viability alterations during ischemia.

Mitochondrial potassium transport: the role of the mitochondrial ATP-sensitive K(+) channel in cardiac function and cardioprotection

Coronary artery disease and its sequelae-ischemia, myocardial infarction, and heart failure-are leading causes of morbidity and mortality in man. Considerable effort has been devoted toward improving functional recovery and reducing the extent of infarction after ischemic episodes. As a step in this direction, it was found that the heart was significantly protected against ischemia-reperfusion injury if it was first preconditioned by brief ischemia or by administering a potassium channel opener. Both of these preconditioning strategies were found to require opening of a K(ATP) channel, and in 1997 we showed that this pivotal role was mediated by the mitochondrial ATP-sensitive K(+) channel (mitoK(ATP)). This paper will review the evidence showing that opening mitoK(ATP) is cardioprotective against ischemia-reperfusion injury and, moreover, that mitoK(ATP) plays this role during all three phases of the natural history of ischemia-reperfusion injury preconditioning, ischemia, and reperfusion. We discuss two distinct mechanisms by which mitoK(ATP) opening protects the heart-increased mitochondrial production of reactive oxygen species (ROS) during the preconditioning phase and regulation of intermembrane space (IMS) volume during the ischemic and reperfusion phases. It is likely that cardioprotection by ischemic preconditioning (IPC) and K(ATP) channel openers (KCOs) arises from utilization of normal physiological processes. Accordingly, we summarize the results of new studies that focus on the role of mitoK(ATP) in normal cardiomyocyte physiology. Here, we observe the same two mechanisms at work. In low-energy states, mitoK(ATP) opening triggers increased mitochondrial ROS production, thereby amplifying a cell signaling pathway leading to gene transcription and cell growth. In high-energy states, mitoK(ATP) opening prevents the matrix contraction that would otherwise occur during high rates of electron transport. MitoK(ATP)-mediated volume regulation, in turn, prevents disruption of the structure-function of the IMS and facilitates efficient energy transfers between mitochondria and myofibrillar ATPases.

Mitochondrial potassium transport: the K(+) cycle

Potassium transport plays three distinct roles in mitochondria. Volume homeostasis to prevent excess matrix swelling is a housekeeping function that is essential for maintaining the structural integrity of the organelle. This function is mediated by the K(+)/H(+) antiporter and was first proposed by Peter Mitchell. Volume homeostasis to prevent excess matrix contraction is a recently discovered function that maintains a fully expanded matrix when diffusive K(+) influx declines due to membrane depolarization caused by high rates of electron transport. Maintaining matrix volume under these conditions is important because matrix contraction inhibits electron transport and also perturbs the structure-function of the intermembrane space (IMS). This volume regulation is mediated by the mitochondrial ATP-sensitive K(+) channel (mitoK(ATP)). Cell signaling functions to protect the cell from ischemia-reperfusion injury and also to trigger transcription of genes required for cell growth. This function depends on the ability of mitoK(ATP) opening to trigger increased mitochondrial production of reactive oxygen species (ROS). This review discusses the properties of the mitochondrial K(+) cycle that help to understand the basis of these diverse effects.

Regulation and critical role of potassium homeostasis in apoptosis

Programmed cell death or apoptosis is broadly responsible for the normal homeostatic removal of cells and has been increasingly implicated in mediating pathological cell loss in many disease states. As the molecular mechanisms of apoptosis have been extensively investigated a critical role for ionic homeostasis in apoptosis has been recently endorsed. In contrast to the ionic mechanism of necrosis that involves Ca(2+) influx and intracellular Ca(2+) accumulation, compelling evidence now indicates that excessive K(+) efflux and intracellular K(+) depletion are key early steps in apoptosis. Physiological concentration of intracellular K(+) acts as a repressor of apoptotic effectors. A huge loss of cellular K(+), likely a common event in apoptosis of many cell types, may serve as a disaster signal allowing the execution of the suicide program by activating key events in the apoptotic cascade including caspase cleavage, cytochrome c release, and endonuclease activation. The pro-apoptotic disruption of K(+) homeostasis can be mediated by over-activated K(+) channels or ionotropic glutamate receptor channels, and most likely, accompanied by reduced K(+) uptake due to dysfunction of Na(+), K(+)-ATPase. Recent studies indicate that, in addition to the K(+) channels in the plasma membrane, mitochondrial K(+) channels and K(+) homeostasis also play important roles in apoptosis. Investigations on the K(+) regulation of apoptosis have provided a more comprehensive understanding of the apoptotic mechanism and may afford novel therapeutic strategies for apoptosis-related diseases.

Defibrillatory action of glibenclamide is independent from ATP-sensitive K+ channels and free radicals

This study investigated whether glibenclamide exerts a defibrillatory action and if this action is mediated by a blockade of ATP-sensitive K+ channels (K(ATP)) or by an anti-free radical mechanism. Aerobically perfused isolated rat hearts were subjected to 10 min of pacing-induced ventricular fibrillation (VF) followed by 10 min of perfusion without pacing (post-VF period), in the presence of solvent (controls), 1 microM K(ATP) blocker glibenclamide, 10 microM K(ATP) opener cromakalim, and their combination, respectively. In controls, pacing-induced VF caused a significant deterioration in cardiac function in the post-VF period. Spontaneous defibrillation was 42%. Glibenclamide improved post-VF cardiac function and resulted in 100% (P < 0.05) spontaneous defibrillation. Cromakalim did not significantly affect post-VF cardiac function and the incidence of spontaneous defibrillation as compared with controls. The combination of the compounds improved cardiac function and resulted in 83% (P < 0.05) spontaneous defibrillation. In separate experiments, 2,5-dihydroxybenzoic acid formation in the perfusate as a marker of hydroxyl radical formation was measured by high-performance liquid chromatography and cardiac superoxide production was assessed by lucigenin-enhanced chemiluminescence during pacing-induced VF. Glibenclamide did not affect hydroxyl radical generation or myocardial superoxide content during VF. The conclusion is that glibenclamide exerts a defibrillatory action and improves post-VF cardiac function in rat hearts and these effects are independent from K(ATP) and free radicals.

SNC-80-induced preconditioning: selective activation of the mitochondrial adenosine triphosphate-gated potassium channel

Pharmacologic preconditioning by delta-opioid agonists occurs via activation of an adenosine triphosphate (ATP)-gated potassium channel (I(KATP)). Opening of mitochondrial I(KATP) confers pharmacologic preconditioning whereas opening the sarcolemmal I(KATP) shortens action potential duration and is proarrhythmic. This study investigated whether SNC-80, a selective delta-opioid agonist, is associated with development of ventricular arrhythmia due to activation of I(KATP). Rabbit isolated hearts were subjected to 12 min of hypoxia and 40 min of reoxygenation after pretreatment with SNC-80 (1 microM, n = 6), pinacidil (1.25 microM, n = 12), or BMS-191095 (6.0 microM, n = 4). Nine additional hearts served as controls. The cytoprotective effects of SNC-80 at a concentration of 1 microM were confirmed using 30 min of regional ischemia followed by 120 min of reperfusion. Ventricular fibrillation (VF) developed in 11 of 12 pinacidil-treated hearts whereas none of the SNC-80-treated (zero of six) hearts developed VF (P < 0.001 compared with pinacidil pretreatment) and zero of four BMS-191095-pretreated hearts developed VF. Similarly, zero of nine control hearts developed VF. SNC-80 reduced infarct size expressed as a percentage of the area at risk from 33 +/- 4% to 14 +/- 3% (P = 0.004) compared with control. SNC-80, which selectively activates the delta-opioid receptor, provided cytoprotection but did not induce VF after hypoxia reoxygenation. The results indicate that pinacidil-induced nonselective activation of I(KATP) results in proarrhythmia that is dependent on activation of the sarcolemmal I(KATP). Selectivity for the mitochondrial I(KATP) is necessary to prevent induction of a proarrhythmic state.

Infarct size limitation by nicorandil: roles of mitochondrial K(ATP) channels, sarcolemmal K(ATP) channels, and protein kinase C

OBJECTIVES

This study aimed to examine:1) whether nicorandil protects the ischemic myocardium by activating sarcolemmal adenosine triphosphate (ATP)-sensitive K(+) (sarcK(ATP)) channels or the mitochondrial K(ATP) (mitoK(ATP)) channels, and 2) whether protein kinase C (PKC) activity is necessary for cardioprotection afforded by nicorandil.

BACKGROUND

Nicorandil is a hybrid of nitrate and a K(ATP) channel opener that activates the sarcK(ATP) and mitoK(ATP) channels. Both of these K(ATP) channels are regulated by PKC, and this kinase may be activated by nitric oxide and also by oxygen free radicals (OFR) generated after mitoK(ATP) channel opening.

METHODS

In isolated rabbit hearts, infarction was induced by 30-min global ischemia/2-h reperfusion with monitoring of the activation recovery interval (ARI), an index of action potential duration. Protein kinase C translocation was assessed by Western blotting.

RESULTS

Nicorandil did not change ARI before ischemia, but it accelerated ARI shortening after the onset of ischemia and reduced infarct size by 90%. A sarcK(ATP) channel selective blocker, HMR1098, abolished acceleration of ischemia-induced ARI-shortening by nicorandil and eliminated 40% of nicorandil-induced infarct size limitation. A mitoK(ATP) channel selective blocker, 5-hydroxydecanoate, abolished the protection afforded by nicorandil without affecting ARI. Cardioprotection by nicorandil was inhibited neither by an OFR scavenger, N-2-mercaptopropionylglycine nor by a PKC inhibitor, calphostin C, at a dose that was capable of inhibiting PKC- epsilon translocation after preconditioning.

CONCLUSIONS

Both the sarcK(ATP) and mitoK(ATP) channels are involved in anti-infarct tolerance afforded by nicorandil, but PKC activation induced by nitric oxide or OFR generation, if any, does not play a crucial role.

Adenosine production and its interaction with protection of ischemic and reperfusion injury of the myocardium

Adenosine exerts cardioprotective effects on the ischemic myocardium. A flexibly mounted microdialysis probe was used to measure the concentration of interstitial adenosine and to assess the activity of ecto-5'-nucleotidase (a key enzyme responsible for adenosine production) in in vivo rat hearts. The level of adenosine during perfusion of adenosine 5'-adenosine monophosphate (AMP) was given as an index of the activity of ecto-5'-nucleotidase in the tissue. Endogenous norepinephrine (NE) activates both alpha(1)-adrenoceptors and protein kinase C (PKC), which, in turn, activates ecto-5'-nucleotidase via phosphorylation thereby enhancing the production of interstitial adenosine. Histamine-release NE activates PKC, which increased ecto-5'-nucleotidase activity and augmented release of adenosine. Opening of cardiac ATP sensitive K(+) (K(ATP)) channels may cause hydroxyl radical (.OH) generation through NE release. Lysophosphatidylcholine (LPC), an endogenous amphiphiphilic lipid metabolite, also increases the concentration of interstitial adenosine in rat hearts, through the PKC-mediated activation of endogenous ecto-5'-nucleotidase. Nitric oxide (NO) facilitates the production of interstitial adenosine, via guanosine 3',5'-cyclic monophosphate (cGMP)-mediated activation of ecto-5'-nucleotidase as another pathway. These mechanisms play an important role in high sensitivity of the cardiac adenosine system. Adenosine plays an important role as a modulator of ischemic reperfusion injury, and that the production and mechanism of action of adenosine are linked with NE release.

Mechanisms by which opening the mitochondrial ATP- sensitive K(+) channel protects the ischemic heart

Diazoxide opening of the mitochondrial ATP-sensitive K(+) (mitoK(ATP)) channel protects the heart against ischemia-reperfusion injury by unknown mechanisms. We investigated the mechanisms by which mitoK(ATP) channel opening may act as an end effector of cardioprotection in the perfused rat heart model, in permeabilized fibers, and in rat heart mitochondria. We show that diazoxide pretreatment preserves the normal low outer membrane permeability to nucleotides and cytochrome c and that these beneficial effects are abolished by the mitoK(ATP) channel inhibitor 5-hydroxydecanoate. We hypothesize that an open mitoK(ATP) channel during ischemia maintains the tight structure of the intermembrane space that is required to preserve the normal low outer membrane permeability to ADP and ATP. This hypothesis is supported by findings in mitochondria showing that small decreases in intermembrane space volume, induced by either osmotic swelling or diazoxide, increased the half-saturation constant for ADP stimulation of respiration and sharply reduced ATP hydrolysis. These effects are proposed to lead to preservation of adenine nucleotides during ischemia and efficient energy transfer upon reperfusion.

[Adenosine production and its role in protection against ischemic and reperfusion injury of the myocardium]

Adenosine exerts cardioprotective effects on the ischemic myocardium. A flexibly mounted microdialysis apparatus was used to measure the concentration of interstitial adenosine and to assess the activity of ecto-5'-nucleotidase (a key enzyme responsible for adenosine production) in in vivo rat hearts. The level of adenosine during adenosine 5'-adenosine monophosphate (AMP) perfusion serves as an index of the activity of ecto-5'-nucleotidase in the tissue. Endogenous norepinephrine (NE) activates alpha 1-adrenoceptors to lead to the activation of protein kinase C (PKC), which, in turn, activates ecto-5'-nucleotidase via phosphorylation, thereby enhancing the production of interstitial adenosine. Histamine-induced release of NE activates PKC, which increases ecto-5'-nucleotidase activity and augments release of adenosine. Nicorandil, a hybrid of an ATP sensitive K+ (KATP) channel opener and nitrate, increases the level of interstitial adenosine via cGMP-mediated activation of ecto-5'-nucleotidase. Opening of cardiac KATP channels may cause hydroxyl radical (.OH) generation. Nitric oxide (NO) facilitates the production of interstitial adenosine, via activation of ecto-5'-nucleotidase. However, singlet oxygen (1O2) is a very powerful oxidant that causes inactivation of ecto-5'-nucleotidase to result in a decrease in the concentration of adenosine in rat heart. Adenosine plays an important role as a modulator of ischemic reperfusion injury. The production and action of adenosine are intimately linked to the release of NE.

Diazoxide protects the rabbit heart following cardioplegic ischemia

K(ATP) channels are present in sarcolemmal and mitochondrial membranes. This study tests the hypothesis that opening mitochondrial K(ATP) channels with Diazoxide (DZ) improves tolerance to cardioplegic ischemia during surgery. Twenty-two rabbit hearts were perfused with Krebs-Henseleit buffer (KHB) on a Langendorff apparatus and underwent 50 min of 37 degrees C global ischemia with St Thomas' cardioplegia (STCP). Hearts were divided into three groups. Ten (control) received no pretreatment. Seven (DZ) received 10 min of 30 microM DZ, a selective mitochondrial K(ATP) opener, in KHB before arrest with STCP containing 30 microM DZ. Five (5-HD + DZ) received 10 min of 100 microM sodium 5-hydroxydecanoate (5-HD), a selective mitochondrial K(ATP) channel blocker, followed by 10 min of 30 microM DZ and 100 microM 5-HD in KHB before arrest with STCP + 30 microM DZ + 100 microM 5-HD. LV developed pressure (LVDP), dP/dt and coronary flow (CF) were measured after 60 min of reperfusion. Diazoxide pretreatment significantly improved the recovery of LV function and coronary flow compared to control (LVDP: 49 +/- 5* vs. 31 +/- 4; +dP/dtmax 927 +/- 93 vs. 507 +/- 85 mmHg/sec*; CF 33 +/- 4 vs. 22 +/- 2 ml/min, *p < 0.05). Mitochondria K(ATP) channel blockade with 5-HD prevented DZ's salutary effect on the recovery of LV and vascular function. Diazoxide pretreatment protects the rabbit heart during cardioplegic ischemia by opening mitochondrial K(ATP) channels. Opening mitochondrial K(ATP) channels may be a new strategy for improving myocardial protection during cardiac surgery.

[In vivo microdialysis technique on rat myocardium]

The microdialysis procedure is a technique that has been established for some years. The heart, however, has several difficulties in the application of this technique. The heart is the beating in vivo, in contrast to other organs. I developed the flexibly mounted microdialysis technique, which involves the synchronized movement of the tip of the probe with the beating heart to reduce tissue injury. With this technique, it is feasible to make stable and long-term measurements of interstitial biological substances. By employing the flexibly mounted microdialysis technique, I will demonstrate that the monitoring of free radical generation and the level of adenosine measured during AMP perfusion that gives an index of the activity of ecto-5'-nucleotidase in the tissue. In the future, this technique will have versatile applications for useful studies to elucidate the actual mechanism in the pathogenesis of heart disorders.

Diazoxide-induced cardioprotection requires signaling through a redox-sensitive mechanism

Diazoxide, a selective opener of the mitochondrial ATP-sensitive potassium channel, has been shown to elicit tolerance to ischemia in cardiac myocytes and in perfused heart. However, the mechanism of this cardioprotection is poorly understood. Because reactive oxygen species (ROS) are recognized as important intracellular signaling molecules and have been implicated in ischemic preconditioning, we examined diazoxide-induced ROS production in adult cardiomyocytes. Cells treated with 50 micromol/L diazoxide showed a 173% increase in ROS production relative to baseline. 5-Hydroxydecanoate was found to attenuate the diazoxide-induced increase in ROS generation. The diazoxide-induced increase in ROS also was abrogated by the addition of either the antioxidant N-acetylcysteine (NAC) or N-mercaptopropionylglycine. We also examined the ability of NAC to block the protective effects of diazoxide in the perfused rat heart. After 20 minutes of global ischemia and 20 minutes of reflow, hearts perfused with 100 micromol/L diazoxide before ischemia showed significantly improved postischemic contractile function relative to untreated hearts (84% versus 29% of initial left ventricular developed pressure, respectively). Hearts treated with diazoxide in the presence of 4 mmol/L NAC recovered 53% of initial left ventricular developed pressure, whereas hearts treated with NAC alone recovered 46% of preischemic function. Using (31)P NMR spectroscopy, we found that, similar to preconditioning, diazoxide significantly attenuated ischemia-induced intracellular acidification and enhanced post- ischemic recovery of phosphocreatine levels, both of which were blocked by cotreatment with NAC. These data suggest that the cardioprotective actions of diazoxide are mediated by generation of a pro-oxidant environment.

Myocardial K(ATP) channels in preconditioning

We are on the brink of harnessing the cell's natural defenses against ischemia and reperfusion injury after years of research into the destructive and protective mechanisms involved. Since the discovery of ischemic preconditioning, the surface receptors and signal transduction pathways underlying this phenomenon have been clarified, but many questions remain about the downstream targets that ultimately protect the cell. ATP-sensitive K(+) (K(ATP)) channels are thought to play a role in protection, but their mechanism of action has been unclear. Accumulating evidence now suggests that the location of the K(ATP) channels relevant to cytoprotection may be on the mitochondrial inner membrane instead of on the sarcolemma of the cardiac cell. This review discusses recent findings and unanswered questions about the role of K(ATP) channels in preconditioning and protection.